Conventionally, a working vehicle such as a tractor provided at is rear portion with a working machine coupling linkage, usually including a single top link and a pair of left and right lower links, through which a working machine can be attached to the vehicle. The working machine coupling linkage is provided with a working machine lifting device which operates an actuator such as a hydraulic cylinder so as to lift and lower a working machine.
The hydraulic actuator (lift cylinder) is installed in a housing mounted on a transmission casing. A piston rod of the lift cylinder is connected to the right and left lower links via lift arms for lifting a working machine so that a working machine is vertically moved by telescopic operation of the lift cylinder.
The above-mentioned working machine lifting device having the single hydraulic cylinder for lifting the working machine is short in strength and rigidity for lifting a wide or heavy working machine. Therefore, Japanese Patent Application Laid Open Gazette No. Hei 8-112006 advocates a technology in which two hydraulic cylinders serving as lift cylinders are externally installed at a rear portion of the vehicle body, and connected to the right and left lower links via lift arms and lift rods or a leveling hydraulic cylinder so as to vertically rotate the working machine.
This working machine lifting device has support points of bottom ends of the externally installed lift cylinders disposed above supporting pivots of the lower links to the tractor, i.e., on the rear side surface of the transmission casing, so as to prevent the externally installed lift cylinders for lifting a working machine from interfering with the working machine coupling three-point linkage including the single top link and two lower links. However, in this arrangement, both ends of each lift cylinder (one is a basal end of its cylinder casing, and the other is a tip of its piston rod) are pivoted on two respective points (one on an intermediate portion of the lift arm, and the other on the rear side surface of the transmission casing) with such a short distance therebetween as to be restrictive in arranging the externally installed lift cylinder having a large capacity or a large stroke.
To enable the large capacity or stroke lift cylinders to be externally installed, an arrangement of lowering the pivots of the cylinders onto the transmission casing is conceivable. To prevent the externally installed lift cylinders from interfering with the working machine coupling linkage, another arrangement of shifting the pivots of the lower links onto the tractor into rear of the transmission casing is also conceivable. However, this arrangement increases the gap between the tractor and the working machine. Such a widened gap is undesirable in view of lengthwise balance of the tractor coupled with a working machine.
Further, each of the proposed arrangements requires that brackets and pins for supporting either the lower links or the bottom ends of the externally installed lift cylinders are disposed on the transmission casing or the like, thereby increasing the number of parts and manufacturing processes.
Further, a conventional valve device for the externally installed lift cylinders has a block bored by vertical holes and horizontal holes for passing oil between ports (detailed in the later “Best Mode for Carrying out the Invention” with reference of FIG. 9), thereby complicating and increasing its manufacturing processes. Additionally, the block must be thick so as to have the oil holes bored therein, thereby being heavy and expensive.
Further, in the conventional working machine lifting device, piston rods of the externally installed cylinders are exposed and subject to sticking or hitting of sand or mud causing damage and corrosion of their sliding surfaces. Therefore, as disclosed in Japanese Patent Application Laid Open Gazette No. 2000-92912, for example, the cylinder covers covering the outer periphery of the respective externally installed lift cylinders are fixed to the piston rods so as to protect the piston rods.
However, each of the disclosed cylinder covers is fixed to a bracket, which is formed on the top of the piston rod of the externally installed lift cylinder. Thus, a marketable hydraulic cylinder cannot serve as the externally installed hydraulic cylinder requiring to be processed for forming the bracket.
However, depending on the degree the cylinder cover may cover the cylinder, it may happen that earth and sand enters the gap between the cylinder and the cylinder cover. The disclosed conventional cylinder cover shaped along the rear, left and right peripheral surfaces of the cylinder is hard to exhaust earth and sand entering the gap between the cylinder cover and the cylinder.